The invention relates to optical communications networks and nodes in optical communications networks. In particular, the invention relates to switches for such networks and nodes, and to the operation and protection of such switches.
The increased demand for communications capacity, particularly as a result of accelerating Internet usage, is driving the need for an all-optical communications network, that is to say, a network in which data is carried at all stages in the form of modulated optical signals by optical fibres. Key components in such a network will be the optical switches which will, for example, operate as signal routing devices at network nodes or junctions.
Of the optical switches currently available, those based on M (micro-electro-mechanical systems) technology tend to be favoured for use in optical networks because of their low loss, low crosstalk and low polarisation dependence. Typically, a MEMS switch comprises N input ports and N output ports defining an Nxc3x97N (for instance 4xc3x974, 32xc3x9732 or 1000xc3x971000) arm with a micro-mirror located at each input port/output port or array intersection. Each mirror may be, for example, electromagnetically, electrostatically or thermally actuatable between first and second positions. In the first position the mirror is out of the path of a free space beam from the corresponding input port; in the second position the mirror intercepts the beam and diverts it to the corresponding output port.
The free space beam could be a discrete optical signal, a multiplexed stream from a single optical fibre or an individual de-multiplexed channel from a multiplexed
Thus, when used in a re-routing situation, say to ensure that a multiplexed stream follows the correct path to its destination, the stream may be re-routed at a MEMS switch included at a network node by actuating the relevant one of the micro-mirrors so as to divert the stream from an input port to the appropriate out port.
A problem obviously arises if a mirror fails, say, because of non-optimised processing it does not operate properly from the outset or it becomes worn through use. Such failure will mean the mirror can no longer be actuated, in which case diversion of a signal at the input port/output port intersection may no longer be possible or the mirror may interfere with any beam from the corresponding input port or to the corresponding output port. In the case of a so-called crossbar switching architecture where there is only one switching device between the inputs and the outputs, which are configured orthogonally, minor failure, although not desirable, may only affect one input/output path. However, in a multistage switching architecture, where there may be multiple interlinked switching devices between the inputs and the outputs, mirror failure may affect multiple outputs. There is, therefore, even in the crossbar switching case, a need to provide protection against mirror failure.
Several mechanisms for prong crossbar configured MEMS switches against mirror failure have to be proposed. A so-called shared protection approach involves adding another row or column of mirrors to the array together with additional external switches. Such an arrangement allows rerouting around a failed mirror.
Dedicated protection may be achieved by using another similar MEMS switch in parallel which can be substituted to compensate for any failures. Additional bypass switches may be used to dictate whether the main or substitute MEMS switch is utilised.
An alternative dedicated protection scheme involves feeding each input to two input ports and taking each output from two output ports, and by having a mirror at each input port/output port intersection, each input signal may follow either of two paths to an output. In other words, the switch is configured in a manner which provides redundancy.
Each of the above identified prior art protection mechanism may add insertion loss, complexity and cost to switching.
An object of the invention is to improve upon the prior art optical switch protection mechanisms.
According to a first aspect, the invention provides an optical switch having a plurality of input ports and a plurality of output ports, wherein associated with each input port there is an input path followed by a free space beam exiting that input port and associated with each output port there is an output path followed by a free space beam entering that output port, mirror means located at the intersection of each input path and each output path, wherein the mirror means comprises a main mirror actuatable to divert a beam from the corresponding input pat to the corresponding output path and at least one back-up mirror for performing the same function as the main mirror.
Thus, each back-up mirror or mirrors provides redundancy, available to take the place of the corresponding main mirror in the event that the main mirror fails. Unlike prior art devices, wherein back-up input/output paths are provided for each input/output path and there is a mirror at the intersection of each input/output and back-up input/output path, in the case of the present invention at least one back-up mirror is provided to operate on the same input/output path as the main mirror. This necessitates an arrangement of the main and back-up mirrors which enables the back-up mirror to take up the same position as the main mirror. The men and the back-up mirrors may be independently and pivotally actuatable. Suitable arrangements are, for example, having independently actuatable main and back-up mirror pairs, each pivotable from out of the input path to intercept the beam from the corresponding input port, and with the minors pivoting about the same pivotal axis. In other words, one mirror may pivot from one side of the axis into position and the other may pivot from the other side of the axis into the same position. The arms on which the mirrors are supported may be so adapted as to enable mounting of the arms on the same pivotal axis. Alternatively, each mirror may pivot about its own pivotal axis with the axes situated to either side of the free space beam. Such an arrangement may be realised by mounting two matching minor arrays face-to-face, with one array providing the main mirrors and the other providing the back-up mirrors.
The main and back-up mirrors may be electrostatically, electro-magnetically or thermally actuated. The switch could be of the MEMS-type. For example, each mirror may be attached to a beam through which a current may be passed to induce heating and expansion. The expansion can be utilized to g a moment and pivoting. Thus, independent control of the current may provide independent control of the mirrors.
The switch may further comprise a main-mirror actuator, a mirror failure detector, a back-up mirror actuator and a failure alarm.
For most applications, one back-up mirror provides adequate redundancy and the main and back-up mirrors may be provided in pans.
Each input part may be supplied with an optical signal from an optical fibre, which could comprise a discrete or multiplexed optical signal or individual optical channels from a multiplexed optical signal stream.
According to a second aspect, the invention provides an optical switch comprising a plurality of input ports and a plurality of output ports defining an array, mirror means located at each am intersection, when the mirror means comprises a main mirror and at least one back-up mirror
According to a third aspect, the invention provides a method of operating an optical switch having a plural of main mirrors each for diverting a free space beam on an input path from an input port along an output path to an output port and each man mirror having at least one back-up mirror for performing the same function as the main mirror, comprising actuating the at least one back-up mirror in the event of failure of the corresponding main mirror
According to a fourth aspect the invention provides a node in an optical communications network, which node includes at least one optical switch according to a first aspect of the invention.
According to a fifth aspect the invention provides a communication network including at least one optical switch according to a first aspect of the invention.